When engine oils become contaminated with the by-products of combustion, their value as a lubricant is greatly diminished. The main contaminate in engine oils during normal combustion is carbon. Diesel engines produce large amounts of carbon, referred to as soot, during combustion and the measurement of the percentage of soot in the diesel oil gives an indication of when the oil should be changed.
Measurement attempts at DC and low frequency AC (ie., below about 1 kilo Hz) fail because the change in conductivity is very small for large changes in the percentages of soot. At high frequencies (ie., in the mega Hz range) the resistive losses due to the soot are measurable even at levels of less than one percent soot. The problem with the high frequency loss measurement is that the sensor, which defines a measurement volume, is a capacitor. Since the sensor has a capacitance associated with its physical shape, there is also a capacitive reactance associated with the sensor. The problem is the capacitive reactance is very low at these high frequencies (ie., on the order of between 1 to 6 hundred Ohms), and the resistance of the soot in oil which is in parallel with the capacitive reactance, is very high (ie., on the order of mega Ohms). There are methods that are used in a laboratory that can make the measurement, but the equipment is expensive and the setup must be nearly ideal (very short leads). The use of a network analyzer or vector voltmeter would be cost prohibitive, and an RF bridge measurement could be used if the resistance and capacitive reactance were near the same values. Thus, such a measurement is very difficult in the real world environment of an operating engine.
Accordingly, what remains needed in the art is a method, applicable to real world engine operation environments, for determining contamination of Diesel engine lubrication oil by measuring the resistance thereof at high frequency.